Because of excellent compatibility with human bone, artificial bone made of apatite can be bonded to the human bone directly. Accordingly, the artificial bone made of apatite has recently been appreciated for effectiveness, finding clinical applications in cosmetic surgery, neurosurgery, plastic surgery, oral surgery, etc. However, artificial ceramic bone such as apatite is not necessarily completely identical with human bone in terms of mechanical properties and physiological properties. For instance, a so-called artificial ceramic bone made only of apatite is harder and more brittle than the human bone. While the human bone is repeatedly subjected to metabolism of absorption and regeneration, the artificial bone made of apatite is not substantially dissolved but semi-permanently remains in human body. The remaining artificial bone breaks human bone at an interface with the human bone, making it likely to cause bone fracture.
Research has recently become active on artificial bone decomposable in the human body, which is closer in composition to human bone than the artificial apatite bone, and various proposals have been made. For instance, JP 11-513590 A discloses a porous body having a network structure, in which collagen and, if necessary, other binders are bonded to hydroxyapatite. Because this porous body is bio-decomposable, human bone is formed in the porous body, and the porous body per se is absorbed in a human body. Accordingly, this porous body can be used for the fixation of vertebra, the filling of bone defects, the repair of fractured bone and, the grafting of periodontal defects, etc.
Because mechanical strength and biocompatibility are substantially inversely proportional to each other in porous bodies comprising apatite and collagen, larger mechanical strength tends to be accompanied by smaller biocompatibility. The porous bodies are thus designed such that these properties are balanced for particular applications. The properties of porous bodies comprising apatite and collagen depend on their porosities to some extent, and the porosity of a porous body can be controlled by the percentage of a liquid (water, an aqueous phosphoric acid solution, etc.), etc. in starting materials. However, because artificial bone usable for various applications should have different properties depending on the applications, the control of porosity is not sufficient.
It is known that mechanical strength and biocompatibility depend not only on the porosity of porous bodies comprising apatite and collagen, but also on their average pore diameters. For instance, the larger average pore diameter the porous body has, the more easily a body fluid, tissues, etc. enter into the pores of the porous body embedded in a human body, and thus the larger biocompatibility the porous body has. The average pore diameter is a factor having large influence on the properties of the porous body comprising apatite and collagen, and there is increasingly larger demand to provide porous bodies with desired average pore diameters recently. However, methods for producing porous bodies comprising apatite and collagen with their average pore diameters controlled have not been known yet.